Double-nulling radiation comparison system



Sept. l5, 1959 C. R. SEBENS DOUBLE-NULLING RADIATION COMPARISON SYSTEM Filed Sept. 27, 1955 INVENTOR.

United States Patent O noULELiNULLmG This :invention `is concerned ywith a :null-typev radiation analysis system;A More particularly, .itris directed .to J a radiation analysis system ywherein.three radiationr-paths are vprovided, two. of .which cooperatively perform the nullingiunction in .a novel..manner;soias'to afford 'avery high degree: of' precision: and :accuracy in ther-analysis measurement.L

Thev solutionfof :problems .which :involve: the measurementio't. very minute ..variations invtheconcentration of a radiation absorptive componentin aV sample mixture become increasingly morefdiicult IAas-the .amplitude of suchlvaria'tions `become smaller'. This vis substantially true lof null-typeias Well as de'iie'cti'onltlype instruments which! measure radiation and 'radiation'` I changes.- Nulltype instruments have certain advantages over deli'ection instruments, but'the measurement ofv the intensity-'0f radiationl in the former type ofj Asystem and AitsA correlation to the `concentration of a radiation ab'sorptivecompo nent Vin v`a A'sample' mixture' has anacc'uracy which 'is`limitedin by the finenes's of-degree to which the nulli'n'g radiation may -beA adjusted, whether the state of 'null `is maintainedl by'a secondary sourcetor a radiation `attenuator. The use of a secondary source toobtain balance between radiation beams necessarily, sacrties" some of the ladvantages of-l a null-type system; suchJ as 'comparativeinsensitivity 'tol 1source uctuati'ons.'

Patented `Sept.15, 1959 of 'a null-type:- radiation analysissystemwhich may -incorporaterthe-present inventionto advantage.

lt'vvillI -b'eassumed therefore for purposes sof illustration :andifexplanation' that an= infra-red absorption system embodies the present invention. In such a systeme-the practice ot the present invention-.contemplates-a Itotal amplitude of nulling infrared. 'energy comprised;V of two components `transmitted through two separate radiation While the sample whichz is undergoing-investigation is placed in amanalysis path' through whichzan amoun of. radiationr is *.transmitted, the totalV nullingsradiation energy .is divid'ed betweenfiirst'and second .nulling paths,I .one `of which: transmitsfthe major :portion .offthe nullingf:energy.l and .remains jixed in vintensity,and4 the other fofs whichs transmits Aa .relatively small'remainingportion.: offthenulling energy. Thel second. nulling pathsis adapted `.t :i\.pass'.'a variable amount yotradiation energy therethrough whichcanbe regulated to maintainA a state of balance or null between the radiation energy passing throughffthe analysis path: and thetotal radiation. energy passing through.' the twornulling paths. The present in- Whil'efasingle source'f'supplyingall the vradiatitimenlployed in a 'system 'overcomes the principal shortcomings o a =dual"radiation' source, it necessitates fon 'the-"other hand, `l the use"'oattenuationmeans"to bring ltheysystem into--a"state'of"null." Inia typical"null"system'the'attenuatorl 'may comprise an "annular-like section`r of radiation opaque "material "having j va lgradually changing "radiali .dii mansion;- Suchi attenuators'are not generally practical, however,n frrange's of attenuation offl'e'ss than '10%l The" fabrication' 'of attenu'ator having" a maximum range VofT 'attenuationfof the order of` 1`1'%j attenuatio'm for instance," with the 'requisite precisin of fcaiib'ration ,and range -stability extremely difficult," if' 'not impossible to achieve. with practicality.' Yet manyfradiation' absorption'ana'ly'sis problems exist which-'require comparable or hner degrees of measurement. The present invention is .directed to the solution of these problems by a novel system which employs conventional `components coacting y.in a unique fashion to bring about 'signicantlyimproved and desirable results. v

The principal object ofthe present invention is ythe precise adjustment'of extremelyl small amounts of the nullng -radiation ina null-type radiation analysissystem.

Another object ofthe present inventinis thel improvement of 'the accuracyof measurement of minute fluctuations in the amount of radiation absorbed by .a'sample`.

A-further object ofthe present invention is the adaptation of known components to av radiation system which affords an order -of. precis-ion of. measurement. markedly superior to .that..attainab1e..by conventional. systems..-. Infrared absorption is one typical basis of'operation v'enti'on-aff'ordsl the advantage off fincreasingzthe accuracy of thefmeasurement of; variation'. in.v total rnulzling energy bya-ffactorwhiclr is `substantially determined by the .ratio of the-total arn'ount of radiation energy passing-'through bothfnulling paths to1 the maximum-.amount 'of'.radiation energy passedby the variable -nulling path.

-bett`er understanding off the-present invention .may belhlad -from ythe followingf'description "and Veiqilan'ati'on of the operation of an` embodiment of `'the present invention, together with 'the accompanying drawings", in which Fig:Y 1 iis" 'aschematic representation of an embodiment of 'thef present invention,

Fig: 2 is an 'illustrationof one type-'of attenuator which may-b'eyusedl in a nondispersion infraredtypefembodiment 'ofthe' :present -inventiom y Figs.. 3-iand 4 are'illust-rations ofv simi-larfattenuators havingdiiierent ranges,

Fig: 5`is :an elevational viewof apparatus embodyinga unitary range-'changer in accordance with the teaching'of the present invention."

The' present'invention contemplatesa system inwhich the sample'vunder investigationis'introduced into a single radatin'pathsuch as'a 'nondi'spers'ive radiation absorption' analysis; system which determines the v absonptive content; of asampl'e mixture,l and 'is bestadapted 'to the solution of isuchlprobl'ems as are not complicated bythe necessity for sensitizing Vand compensating the radiation paths to effect.'acceptable'results'. In accordance with the teachingof thepresent invention, thesampleroccupies a portion of the `iirst'radiationpath-and a second radiation. path may be employed to' pass' apredomina'nt portion of the nullinglenergy'required which may be'90% of the total nulling energy, for instance. A third radiationpathv which is calibrated and adjustably variable supplies the remaining 10%" of the 'nulling 'energy' required to effect system balance and maintaina state ofl Infsuch'a system the variation in the amount of energy whichis requiredtomaintain a state of 7nullis a measure of the variation in. absorption by the sample under investigation. The present invention is particularly suited' to the precise measurement of verysma-ll incremental variations in absorption by the sample under investigation and lends itself readilyfto a', form which will accommodate a con-` tinuously owing, sample wherein it may be desired to measure extremely minute traces of a particular radiation absorptiye component.

As shown. in Fig. l, a typical embodiment of the present invention may comprise a source S'providingrthree beams of radiation which are initially of 'equal intensity and' pass along three separate radiation paths 10, 11, and 12. One of these paths has a sample chamber 13 which is adapted to receive the sample undergoing investigation. The sampleV may be a discrete portion of a iiuid mixture, for instance, or it may be a continuous flow Vof uid mixture tapped from a process stream. The fluid. mixture may include any of numerous gases or liquids which do not completely absorb the radiation employed in the system.

The system is preferably of the A.C. type and. accordingly a chopper 14 which is opaque to the radiation employed in the system may be positioned and arranged to interrupt the radiation transmitted through the three paths 10, 11, and 12. The chopper 14 is driven at a constant speed by a motor 15 and is shaped so that the two nulling paths 11 and 12 are blocked 'in unison while radiation is allowed to pass along the analysis path 10; conversely, Vthe analysis path 10 is blocked while the radiationis allowed to pass along both nulling paths 11 and 12. All three paths are arranged to converge their respective radiations upon a common detector 16 which produces a signal commensurate with the variations of its received radiation.

The signal produced by the detector 16 is amplified in a preamplilier 17 and an amplifier 18, whence it is fed to a phase sensitive demodulator 19. The motor 15 which drives the chopper device 14 also drives a generator 20 to produce an alternating signal in synchronism with the radiation chopping operation. The demodulator 19 produces a rectified signal proportional to any disparity which may exist between the radiation energy passed by the analysis path 10 and the total of the radiation energies passed by the two nulling paths 11 and 12. The output of the demodulator 19 is filtered in a filter 21 and fed to a servoamplilier 22 which in turn provides a servomotor 23 with appropriate power to actuatc an adjustably Variable attenuator 24 positioned in one of the nulling paths 11. The system is arranged and adjusted so that the amount of radiant energy passing through the adjustable nulling path 11 is regulated in response to the demodulator signal; the total amount of nulling energy passing through the two nulling paths 11 and 12 is varied so as to be constantly in balance with the amount of energy passed by the analysis path 10. Thus, as the amount of absorptive component in the analysis path 10 increases or decreases within the sample, a consistent balancing change takes place in the adjustably variable nulling path 11. The amount of such attenuation may he correlated to an electrical signal by mechanically linking the shaft 25 of the attenuator 24 to a wiper 26 of a potentiometer 27. The signal thus picked off actuates a recorder 28 after appropriate filtering in a filter 29.

In accordance with the teaching of the present invention, the nulling path 12 which passes a xed amount of radiant energy supplies a substantially greater proportion of the total nulling energy than does the adjustably variable nulling path 11. Thus, in a typical embodiment the fixed nulling path 12 may supply 90% of the nulling energy required to maintain the system in a state of balance, whereas the nulling path 11 supplies radiant energy in an adjustable amount substantially of the order of 10% of the total nulling energy required to maintain the system in a state of balance. Any desired ratio between the two nulling paths may be effected by conventional trimmers, such as those shown at 30, 31, and 32 in the radiation paths 10, 11 and 12 respectively of Fig. l.

One of the prime advantages of the present invention is that the amount of variation in the adjustable nulling path 11 may be Very precisely regulated and accurately measured. To illustrate the diiculty of precise measurement of small increments of radiant energy by conventional systems, reference is made to Figs. 2, 3, and 4 in which several variable attenuators typical of those employed in the infrared instrumentation art are illustrated. These attenuators are intended to be used in the 4 manner of the attenuator 24 illustrated in Fig. 1. It will beseen that the attenuator illustrated in Fig. 2, for instance, effects change in the amount of energy which it intercepts by a quite noticeable variation in the deviation of its periphery from concentricity. The attenuator shown in Fig. 3 has considerably less deviation, and that shown in Fig. 4 has even less deviation and, in fact, the variation in the width of the annulus of the latter attenuator is barely discernible within small segments of its periphery. The variable attenuator shown in Fig. f4 is, however, a 10% attenuator which, when used in a conventional system, will vary the energy passing through a radiation path over a 10% range; for example, the total range of variation atorded might be to 100% if only one nulling path is employed in a radiation analysis system.

Using a 10% attenuator in the conventional two-beam system, it is most diicult to accurately measure incremental variations of the order of a small fraction of 1% in the concentrationof radiation absorptive component in a sample. It would seem to follow quite logically that the diiculty of such measurements might be overcome by the use of an attenuator which has a maximum range of variation of the order of 1%, but the problems involved in fabricating such an attenuator are manifest to .those skilled in the a'rt. VThe use of an attenuator similar to the type illustrated' by Figs. 2, 3, and 4 having a periphery varying from concentricity so as to alord a full range adjustment of the order of 1% or'2% or less, would likely render measurement very unreliable so far as accurary is concerned. This is evident from the fact that the variations of its periphery from concentricity would of necessity approach the infinitesimal within small increments, Linearity within minute increments would therefore be unreliable. This problem is further aggravated by the necessity of the entire range of the attenuator being consistently linear. It is readily apparent that any deformity suffered by such an attenuator in handling or use might be a source of major error in the system, however slight or imperceptible the damage may seem.

In accordance with the teaching of the present invention, a substantially greater proportion of the nulling energy passes through a first nulling path,A and adjustment of the total nulling energy may be accomplished with a 10% attenuator, for example, such as is illustrated in Fig. 4. Accordingly, the maximum variation in percent attenuation which it is possible to achieve will be approximately 10% divided by the ratio of the total nulling energy of the, system to that which may be passed through the second nulling path. Thus, if 10% of the nulling energy passes through the variable nulling path and a 10% attenuator is used to vary and adjust the lamount of nulling energy required to maintain the system `in arstate of balance, a greater order of precision may be obtained in that the variably adjustable nulling energy may be adjusted ythrough 10% of its maximum range, or 41% of the total nulling energy.

'Ihis greaterrdegree of precision afforded by the present invention prevails with equal effectiveness ,for incremental variations, which are but a very small fraction of the maximum variation of attenuation possible. Minute fractional variations may be measured with an accuracy enhanced to an extent consistent with the divis ion of the total nulling energy between the two nulling paths. Thus, in radiation analysis problems wherein a relativelyV very small variation of a particular radiation absorptive component must be discerned, Ithe present invention affords a system whereby aV conventional attenuator maybe employed to achieve precision of adjustment and accuracy. of measurement far beyond its capabilities inconventional systems, particularly as `tosmall fractional variations.

In accordance with the teaching of the present invention the sum of the energy passed by the nulling beams has a maximum equal to the maximum energy passed by the analysis beam. Since the total nulling energy may be divided between the two nulling beams as desired, the system of the present invention therefore aords the further feature of a convenient and rapid means of changing range of the instrument by changing the ratio of the nulling energies passed by the two nulling paths.

As illustrated in Fig. 5, a preferred type of range changer may comprise an opaque member 33 having appropriately spaced apertures 34 and 35 positioned with respect to the two nulling paths 11 and .12 so that the total amount of energy passed by the nulling paths is always equal to the maximum amount of energy passed by the analysis path .10. The range changer 33 is movable laterally across the principal axis of the beams and one simple adjustment by means of a calibrated screw mechanism 36 connected to the range changer 33 and mounted on the instrument frame 37 determines the relative division of nulling energies between the two paths 11 and 12 and therefore establishes any desired range of nulling adjustment. It may be readily appreciated by those skilled in the art that an embodiment of the present invention which incorporates a unitary member to selectively vary the range of the system in a manner as illustrated in Fig. 5 requires little or no zero adjustment, since the total aperture areas 34 and 35 of the unitary range changer 33 passes a total nulling energy which is always equal to the maximum analysis path energy regardless of the range selected.

Another advantage of the unitary range changer 33 is that it affords variation of range of any desired value rather than several arbitrary settings. Accordingly, the range changer of the present invention may be calibrated for a continuusly variable series of adjustments.

Since many changes could be made in the specific combinations of apparatus disclosed herein and many apparently diierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as being illustrative and not in a limiting sense.

l claim:

l. In a null-type analyzer, the combination of a source of radiation, a detector for receiving radiation from the source and responding thereto, means defining an analysis path, means connected to said analysis path for introducing a sample therein, rst and second nulling paths for passing radiation traveling from the source to the detector, said rst and second nulling paths being isolated in fluid ow relationship from said analysis path, means for passing a fixed amount of radiation through said rst nulling path, said radiant energy being of substantially greater intensity than that passing through said second nulling path, and means for varying the amount of energy passed by said second nulling path so that the sum of the energy passing through said nulling paths equals that passing through said analysis 5 path.

2. In a null-type analyzer, the combination of a source of radiation, a detector for receiving radiation from the source and responding thereto, means dening an analysis path, means connected to said analysis path for 10 introducing a sample therein, rst and second nulling paths for passing radiation traveling from the source to the detector, said first and second nulling paths being isolated in uid flow relationship from said analysis path, means for passing a fixed amount of radiant energyl through said rst nulling path, and means for varying the amount of energy passed by said second nulling path so that the sum of the energy passing through said nulling paths equals that passing through said analysis path, the maximum amount of energy passing through said second nulling path being less than one-tenth of that passing through said rst nulling path.

3. In a null-type analyzer, the combination of a source of radiation, a detector for receiving radiation from the source and responding thereto, means defining an analysis path, means connected to said analysis path for introducing a sample therein, two separate nulling paths ,for passing radiation traveling from the source to the detector, said first and second nulling paths being iso- -lated in tluid ow relationship from said analysis path, radiation opaque means positioned adjacent said nulling paths and having openings therethrough for permitting the passage of radiation, means for adjusting the position of said last-named means relative to said nulling .paths whereby to vary the ratio of maximum radiation passed by said nulling paths, means for periodically blocking said paths, said nulling paths being blocked simulptaneously and out of phase with said analysis path, means for adjustably varying the amount of radiation energy passing through only one of said nulling paths, and means responsive to the signal of said detector for controlling the energy passing through said adjustable nulling path so that the sum of the energy passing through both of said nulling paths is equal to that passing through said analysis path.

References Cited in the file of this patent UNITED STATES PATENTS 2,688,089 Williams Allg. 31, 1954 2,688,090 Woodhull et al Aug. 3l, 1954 2,698,390 LiStOn Dec. 28, 1954 2,718,597 Heigl et al Sept. 20, 1955 2,754,424 Woodhull et al. July 10, 1956 

